If your iPhone cannot connect to your carrier but can connect to other carriers, the SOS will be shown.
That means your carrier is having intermittent network or tower or coverage issues in your area.
If you see SOS, No Service, or Searching on your iPhone or iPad - Apple Support
Options for communications include setting up satellite communications on iPhone 14 and newer, switching carriers to or adding an eSIM for a second carrier with better terrestrial coverage in your typical locations, satellite Wi-Fi with carrier Wi-Fi calling, direct-to-satellite where offered, and a microcell if your carrier offers that. Or a Garmin inReach or other satellite communications device.